Steve is on the right track. You'll blow the engine long before you get to the breaking point of the driveshaft and rearend. Besides the gear swap, another option might be to look into a Ranger Overdrive. It's basically an auxilliary transmission that bolts between the bellhousing and the main trans. They're available in Overdrive and Underdrive versions, both of which will give you 8 forward gears to choose from. It does moove the transmission back about 7".
